Flexible conductive disc electrode



United States Patent T. O. Paine Deputy Administrator of the NationalAeronautics and Space Administration with Respect to an Invention of;

Frank B. Ramme, Lancaster, Calif.

Appl. No. 760,389

Filed Aug. 29,1968

Patented Dec. 15, 1970 Inventors FLEXIBLE CONDUCTIVE DISC ELECTRODE3,029,808 4/1962 Kagan 128/206 3,170,459 2/1965 Phipps et a1. 128/2063,426,746 2/1969 Seamans,Jr.. 128/206 3,464,404 9/1969 Mason 128/206Primary Examiner-William E. Kamm Attorney-Monte F. Mott, J. H. WardenandG. T. McCoy ABSTRACT: A dry, electrode featuring a wire sandwichedbetween two discs cut from sheets of a flexible conductive material. Theelectrodes are made by mixing silver powder with a cement, diluting themixture with a suitable solvent for the cement and applying it in a thinlayer to a surface from which it can be removed as a film when dry;cutting or shaping the dried film to form the size electrodes desired;placing a wire lead on the cut film piece; applying a small amountofsolvent to the film surface having the wire thereon; and pressing asecond piece of the dried film thereto to form the electrode of theinvention.

PATENTEI] mm 5 m 3547;105

FIG. la

INVENTOR FRANK B. RAMME ATTORNEYS FLEXIBLE CONDUCTIVE DISC ELECTRODEORIGIN OF INVENTION 7 The invention described herein was made in theperformance of work under a NASA contract and is subject to theprovision of Section 305 of the National Aeronautics and Space Act of1958, Public Law 83-568 (72stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention isin the field of dry electrodes. More particularly the invention is inthe field of dry electrodes for use in physiological monitoring ofindividuals.

2. Description of the Prior Art Prior to the herein invention there havebeen developed dry electrodes for application to individuals. Theelectrodes were for physiological monitoring, such as forelectrocardiograms and electroencephalograms. This work is described inNASA Technical Note D-34l4, May 1966, Dry Electrodes for PhysiologicalMonitoring," by Charles W. Patten, Frank B. Ramme and James A. Roman.This prior method involved forming a solution of conductive electrodematerial to be sprayed on to the individual. The solution was comprisedof powdered silver in a cement, such as Duco cement made by the DuPontCompany, which is cellulose nitrate dissolved in a suitable solvent. Theviscosity was controlled by adding more solvent for the cement such asacetone, so that the mixture could be readily sprayed through anatomizer. Upon evaporation of the solvent from the sprayed mixture, adry conductive electrode of the silver disposed in the cement wouldreside on the person sprayed.

In the practice of this prior technique, a small amount of conventionalelectrode jelly was wiped on the area of the skin to which the electrodewas to be applied. The lead wire utilized was Teflon insulated copperwire that had a pin connector on one end for eventual connection to thereadout instrument, while the other end of the wire was stripped with asmall epoxy ball fastened to the end tip. The stripped end of the wirewith the ball thereon was placed against the nozzle of the spray gunutilized. The nozzle was then placed against the skin and centered overthe jelly coat. A coating of the conductive cement mixture was thensprayed onto the skin to form about a half-dollar size electrode andcapturethe lead wire in the cement. The same spray gun was then utilizedby a valve control to spray air only against the dried electrode torapidly dry it.

Care had to be taken to withdraw the gun about two seconds after thestart of the air dry to prevent the gun from adhering to the sprayedarea on the skin. The aforegoing steps were repeated for each electrodeto be applied to the individual. Normally three different electrodes areapplied, for example, in taking electrocardiograms. Additionally, forbest results, the wire should not contact the skin. Thus, a thin coat ofthe conductive mixture is preferably sprayed and dried before the wireis pressed onto the skin, then the spraying continued to place a furthercoat over it and the wire. As can be seen the aforegoing is a fairlytime consuming process with certain critical steps. Additionally, theequipment to spray the conductive mixture is relatively expensive and issubject to fouling or clogging and the like. Thus, though the concept offorming electrodes in situ on the body would appear to be expeditiousand advantageous, there are several drawbacks.

Thus, an object of this invention is to provide a dry electrode which isprefabricated prior to application to subject.

Another object of this invention is to provide a dry electrode which isreadily and easily applicable to a subject.

SUMMARY OF THE INVENTION The herein invention comprises mixing asuitable cement, such as a cellulose nitrate type cement, with silverpowder in about a 50 50 weight ratio, together with enough solvent toprovide a fluid mixture to be sprayed. The solvent preferably used is anorganic material such as acetone. The mixture is then sprayed on a sheetof inert material such as polyethylene to a desired thickness. Afterdrying, the conductive cement is detached from the plastic sheet andplaced upon a sheet of material such as rubber so that discs can bepunched from the cement. Utilizing a conventional punch, discs of anychosen diameter can then be provided from the dried sheet of conductivecement. To form the completed electrode a wire having a stripped end isplaced on one disc resting on a sheet of inert plastic, a drop or two ofacetone or other suitable solvent for the cement is then placed on thedisc, and a second disc immediately then pressed over the wire to theunderlying first disc to form a sandwich about the wire. The, product isagain allowed to dry. The final dry electrode thus has two discssandwiched about the lead wire. The two discs, of course, are heldtogether by the adhesive action due to the action of the solvent and canthen be readily applied to the subject by wiping a desired section ofthe subjects body with acetone and pressing the dried disc onthat area;leaving it there until the acetone evaporates. It is believed theinvention will be better understood from the following detaileddescription and drawings in which:

FIGS. la-ld schematically depict the steps of forming the dry electrodeof this invention;

FIG. 2 is a top view of a dry electrode of the invention;

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to the drawingthere is seen in FIG. la, a sheet of plastic 11 with a nozzle 13 from anatomizer or like assembly disposed adjacent thereto. The nozzle 13sprays a conductive cement on the plastic 11. The conductive cement canbe of a variety of materials. It has been found that the metal portionof the cement should be preferably silver powder. The metal should beconductive. However, it should not readily oxidize since this interfereswith the electrical properties. For this reason, silver is preferableover a material such as copper. Another metal that could be used isplatinum. However, it is considerably more expensive.

The metal powder is initially mixed with a cement material which shouldbe fast drying and adherent to the skin. It is apparent that the basiccement material should also be inert to The organic solvent which servesthe purpose of decreasing.

the viscosity of the cement so that it can be easily sprayed from theatomizer head 13, should also, of course, be a material that will betolerated by the body as well as rapidly evaporating. Acetone is foundto be preferable as a solvent material to serve the aforegoing purposes.However, otherorganic materials such as methyl ethyl ketone, butylacetate, amyl acetate, and the like can be utilized in addition to or inplace of acetone.

To form the initial mixture, silver or other metal used is added to thecement on the weight ratio of to percent of silver to the amount ofcement present. The amount of acetone or other suitable solvent added tothe mixture depends solely upon the spray apparatus utilized. Onlyenough acetone should be used so that an adequate spray can be obtained.Too fluid a material is not readily handled, and the thickness cannot beeasily controlled when applied to the plastic sheet 11. The plasticsheet 11 should'be of a material that is inert to the cement so it willnot adhere to it. For example, a typical plastic would be polyethyleneplastic. I

To control the thickness of the conductive cement applied to the plasticsheet 21, one can maintain the surface area of a I portion of the sheetat a given size, and vary the amount of material applied to thatsurface. Thus, by applying twice the quantity of the conductive cementto a given surface area, one will double the thickness of the finalproduct. The thickness is not critical. The cement product can vary fromabout 7 mils to one that is several times as thick. 7

After the conductive cement has been sprayed onto the surface ll, it isallowed to dry. This can be aided by means of a fan to circulate airover the surface, aiding in the evaporation of the solvent used. Uponcompletion of the drying, the conductive cement is then peeled, as inFIG. 1b, from the base 11. This produces a continuous sheet of a desiredthickness of disc elements from the sheet 15 as seen at the remainingholes 21. The discs removed are preferably the size of a half-dollar butcan obviously be of any suitable diameter.

To form the complete electrode element, two discs, as shown in FIG. 1d,are utilized. A first disc 23 has a drop or two of solvent applied toits surface. The lead wire 25 is then laid upon the disc as shown. Asecond disc 27 is then placed on top of the first disc 23, sandwichingthe wire there between, and pressed upon the first disc 23. The solventrapidly evaporates, leaving a solid electrode member. The last operationcan be performed while the bottom disc 23 is once again resting on asheet of plastic material similar to or the same as the sheet used inthe initial spraying step shown in FIG. la.

FIGS. 2 and 3 combined show a completed unit. The insulated lead wire 25preferably is exposed along the portion 29 that extends between the twodiscs 23 and 27, and is silver plated thereat to provide a nonoxidativesurface in this region, as well as improve the conductivity from theconductive cement to the wire. The wire 25 is then capable of beingconnected to a suitable plug which can also receive the other electrodesutilized. The plug in turn can be attached to a line fed into anelectrocardiogram apparatus or the like.

To use the completed dry electrodeas shown in FIGS. 2 and 3, a portionof the body to which the electrode is to be applied is wiped or dabbedwith the solvent used for the cement, such as, for example, acetone. Theelectrode is also wetted with solvent, then immediately pressed upon thewet area of the body and held for a second or two until the electrodehas started to adhere. It has been found that the cement is notconductive when wet with solvent. Thus, one of the advantages of theherein invention as compared to the priorart spraying device is that theelectrode when applied is basically dry. This small layer of solvent isabsorbed into the skin as well as the dry electrode at an extremelyrapid rate. Thus, the electrode is ready for use much sooner than in thespray-on device which takes longer to dry.

ln order to remove the electrode from the body, one can simply peel itoff in the manner of an adhesive bandage, or alternatively, where thearea might be covered with hair or the like, the entire electrode can bedissolved when wiped with the liquid solvent. The invention will befully understood from the following example.

EXAMPLE To form a conductive cement mixture capable of being sprayed, 43grams of Duco household cement, manufactured by E. I. DuPont de Nemours& Company, which is a cellulose nitrate cement, containing acetone andbutyl acetate as solvents therefor together with a high boiling esterplasticizer, was mixed with 43 grams of silver powder and milliliters ofacetone. The mixture was sprayed onto a sheet of polyethylene plasticover a square area of 6 by 6 inches. This produced a thickness ofapproximately 7 mils over that area. The conductive layer was thenpermitted to dry into a pliable nontacky solid surface on the plasticsheet. The conductive layer was then peeled off of the underlyingIplastic and laid upon a sheet of neoprene rubber. DISCS of t rec-fourthsinch diameter were then cut with a punch from 'the sheet of conductivecement. To form the completed electrode, a cut disc was then laid upon asheet of polyethylene. Two drops of acetone were placed on the topexposed surface of the disc and a lead wire, which was silver plated inthe region of contact with the disc, was laid upon it. Immediately, asecond disc a was then placed on top of the first one sandwiching thewire therebetween. Pressure was applied to the two discs for a second ortwo to allow the acetone solvent to thoroughly blend the two discstogether about the wire. The electrode together with its lead wire wasthen ready to be attached to a suitable plug for eventual connection toa readout apparatus.

It should be apparent that the invention can be practiced with obviousmodifications to the process described. For example, the initial mixtureof conductive cement can be applied by methods other than spraying. Itcan be rolled onto a base or applied through a dipping. In other words,various techniques can be used to obtain a dry film of the conductivecement to be cut into discs.

Additionally, the conductive cement does not have to be removed from thebase prior to punching the discs. It is just believed to facilitate theseparation of the cement from the plastic sheet if it is accomplishedbefore the discs are punched.

I claim:

1. A dry electrode for application to the body comprising:

two discs of cement that is soluble in anorganic solvent,

said cement having sufficient metal powder dispersed therethrough toprovide electrical conductivity;

an electrical lead wire; and

means securing said discs to each other sandwiching one end of said wiretherebetween.

2. The electrode of claim 1, wherein said means for securing said discscomprises an amount of solvent for said discs sufficient only to softena surface layer of said discs.

3. The electrode of claim l wherein said cement is selected from thegroup consisting of cellulose'nitrate and cellulose acetate, and whereinsaid metal powder is silver.

4. The device of claim 3 wherein the portion of said wire sandwichedbetween said discs is silver plated.

